Fabric washing powders containing major quantities of soap are favoured by some consumers because of good detergency, and the tendency to leave clothes feeling softer than those washed with powders based on synthetic detergent-active compounds. Soap also has environmental advantages in that it is fully biodegradable, and is a natural material derived from renewable raw materials.
There is, however, a technical problem with soap in that it is not easy to obtain satisfactory dissolution, particularly at the low temperatures favoured in today's low-energy washing machines. There are two aspects of this problem: first, poor wetting characteristics can lead to clotting or gel formation; and secondly, even when wetting characteristics are satisfactory, there remains the problem of the inherent poor solubility of soap, particularly at low temperatures.
GB 2 034 741B (Unilever) discloses a soap powder composition of improved inherent solubility. The soap powder comprises, in addition to builder salts and other conventional ingredients, from 15 to 60 wt % of a defined soap blend having a low Krafft temperature (below 25.degree. C.), derived from a C.sub.12 -C.sub.22 fatty acid mixture comprising
(i) from 5 to 60 wt % of one of more saturated or unsaturated fatty acids having 14 or fewer carbon atoms,
(ii) from 5 to 32 wt % of one or more saturated fatty acids having more than 14 carbon atoms,
(iii) from 35 to 90 wt % of one mor more unsaturated fatty acids having 14 or more carbon atoms.
This soap blend has been found to give good detergency with improved inherent solubility compared with standard coconut/tallow blends. However, the wetting characteristics of powders containing this blend have not proved ideal, the formation of clots being especially noticeable when the powders are used in a top-loading washing machine at a low wash temperature.
EP 340 013A (Unilever) discloses detergent powders based on synthetic detergent-active compounds (notably alkylbenzene sulphonate) and zeolite, granulated and densified to bulk densities above 650 g/liter in a high-speed mixer/granulator having both a stirring action and a cutting action, for example, the Fukae FS series mixer/granulator.
It has surprisingly been found that such granulation and densification of soap powder based on low-Krafft-temperature soap blends gives substantially improved wetting and dispersion characteristics without loss of other desirable properties, as well as substantially better powder properties (bulk density, flow, compressibility).
JP 62 086 099A (Nippon Oils & Fats) discloses a process for the manufacture of a composite soap powder (the term used in Japan for powders containing both soap and synthetic detergent-active materials, when the soap amounts to less than 70 wt % of the total detergent-active material). In the process, fatty acid soap (in the form of chips), synthetic detergent-active agent and inorganic and/or organic builders are disintegrated and mixed in a lateral-type mixer/granulator (a cylindrical housing containing two types of stirrers), to give a high-bulk-density product consisting of spherical granules even though the content of non-soap detergent is high. The product typically contains 40-55 wt % soap, 5-20 wt % nonionic surfactant (7-15 wt % exemplified) and 25-50 wt % builder. The choice of soap is apparently not critical; sodium beef tallow soap, potassium beef tallow soap and a mixed soap (coconut/soybean/beef tallow 4:1:15) are exemplified; and there is no disclosure of low-Krafft-temperature soap.